Microneedles are attractive for delivery of certain therapeutics. These needles are virtually painless because they do not penetrate deep enough to touch nerves because, unlike traditional syringes and hypodermic needles, microneedles typically only penetrate the outermost layer of the skin. Additionally, shallower penetration reduces the chance of infection and injury. Furthermore, microneedles facilitate delivery of an exact dosage of a therapeutic which allows use lower doses in treatments.
Microneedles often require a manufacturing process that allows mass production at lowest cost, and as a consequence, shortest possible cycle time. In order to have proper transcription of mold texture and shape to the molded part, high flow may be necessary, especially having low viscosity at extremely high shear rates. Furthermore, good release from the production mold is important to reduce cycle time to improve the cost efficiency. Finally, these needles should have good strength to prevent breaking of the microneedle during usage.
Current materials of choice for microneedles are liquid crystalline polymers, polycarbonate, and polyetherimide. These materials all have certain limitations for microneedle applications. Although liquid crystalline polymers have excellent flow, their mechanical properties depend on the flow direction and needle strength may suffer because of this. Polyetherimide is known for its excellent strength, but the flow of this material is far from optimal and very high temperatures are required to be able to mold this polymer into the desired fine features of a microneedle mold. Polycarbonate is flexible in molding conditions, easily formable and has acceptable mechanical properties for the application in microneedles. At high shear rates though, around and beyond 106 inverse seconds (s−1), a plateau value in viscosity may be reached. In some cases, a further increase in shear rate even causes shear thickening behavior which makes filling the fine microfeatures in microneedles molds more difficult. The shear thickening phenomenon is thought to be caused by molecular orientation in the melt.
The fine featured microneedles require excellent mold release properties in order not to get damaged. This can be achieved by cooling the mold deeper than for typical molding operations, but requires an increased expense of energy cost and cycle time and is in general an uneconomic solution. Alternatively, there is a variety of commercial mold release additives such as pentaerythritol tetrastearate (PETS) and waxes that improve mold release behavior, but (traces of) such compounds may remain in the mold and build a deposit after a number of molding cycles which may directly impact needle shape and sharpness.
These and other shortcomings are addressed by aspects of the present disclosure.